A developing device which is used for an image forming apparatus such as a copy machine or a printer carries out development using a two-component developer. Generally, a developing device using a two-component developer is replenished with toner as it is consumed by development. However, in such a developing device, while the device is replenished with toner, the capability of the carrier is lowered and the capability to charge the toner is deteriorated.
Therefore, conventionally, for example, JP-A-6-348134 discloses a trickle development system to restrain deterioration of the toner charging capability of the carrier. In the trickle development system, a developing container is replenished with new toner and an excess amount of developer is discharged from a discharge port. Thus, the deteriorated carrier is replaced by the new carrier.
However, in the above conventional developing device, the quantity of carrier replenishment into the developing container is decided in accordance with the quantity of consumed toner. Therefore, practically, replenishment with a required quantity of carrier may not be done even though the carrier is seriously deteriorated. If the quantity of carrier replenishment is not enough in this manner, insufficient charging of toner occurs. Consequently, the image quality of the developed toner image may degrade or trouble such as ground fogging may occur. Meanwhile, because of the replenishment with the carrier, the carrier that is not practically deteriorated may be discharged. In this case, the carrier is wasted.
Thus, the developing container is properly replenished with the carrier in accordance with the degradation of the carrier. Consequently, the carrier is not wasted. Also, a sharp developed image is provided and trouble such as ground fogging is prevented to improve the image quality. Development of an image forming apparatus that can realize this is demanded.